REVISION 10 3/18/15 7 LOW VOLTAGE, 1:27 CLOCK DISTRIBUTION CHIP
MPC941 DATA SHEET
Power Consumption of the MPC941 and Thermal
Management
The MPC941 AC specification is guaranteed for the entire
operating frequency range up to 250 MHz. The MPC941
power consumption and the associated long-term reliability
may decrease the maximum frequency limit, depending on
operating conditions such as clock frequency, supply voltage,
output loading, ambient temperture, vertical convection and
thermal conductivity of package and board. This section
describes the impact of these parameters on the junction
temperature and gives a guideline to estimate the MPC941
die junction temperature and the associated device reliability.
For a complete analysis of power consumption as a function
of operating conditions and associated long term device
reliability, please refer to the Freescale application note
AN1545. According the AN1545, the long-term device
reliability is a function of the die junction temperature:
Increased power consumption will increase the die
junction temperature and impact the device reliability
(MTBF). According to the system-defined tolerable MTBF,
the die junction temperature of the MPC941 needs to be
controlled, and the thermal impedance of the board/package
should be optimized. The power dissipated in the MPC941 is
represented in equation 1.
Where I
CCQ
is the static current consumption of the
MPC941, C
PD
is the power dissipation capacitance per
output. C
L
represents the external capacitive output
load, and N is the number of active outputs (N is always 27 in
case of the MPC941). The MPC941 supports driving
transmission lines to maintain high signal integrity and tight
timing parameters. Any transmission line will hide the lumped
capacitive load at the end of the board trace, therefore,
C
L
is zero for controlled transmission line systems and can be
eliminated from equation 1. Using parallel termination output
termination results in equation 2 for power dissipation.
In equation 2, P stands for the number of outputs with a
parallel or thevenin termination. V
OL
, I
OL
, V
OH
and I
OH
are a
function of the output termination technique, and DC
Q
is the
clock signal duty cyle. If transmission lines are used,
C
L
is
zero in equation 2 and can be eliminated. In general, the use
of controlled transmission line techniques eliminates the
impact of the lumped capacitive loads at the end lines and
greatly reduces the power dissipation of the device.
Equation 3 describes the die junction temperature T
J
as a
function of the power consumption.
Where R
thja
is the thermal impedance of the package
(junction to ambient), and T
A
is the ambient temperature,
according to Table 7, the junction temperature can be used to
estimate the long-term device reliability. Further, combining
equation 1 and equation 2 results in a maximum operating
frequency for the MPC941 in a series terminated
transmission line system.
T
J,MAX
should be selected according to the MTBF system
requirements, and Table 7, R
thja
can be derived from Tab le 8.
The R
thja
represent data based on 1S2P boards. Using 2S2P
boards will result in a lower thermal impedance than indicated
below.
If the calculated maximum frequency is below 250 MHz, it
becomes the upper clock speed limit for the given application
conditions. The following eight derating charts describe the
safe frequency operation range for the MPC941. The charts
were calculated for a maximum tolerable die junction
temperature of 110C (120C), corresponding to a estimated
MTBF of 9.1 years (4 years), a supply voltage of either 3.3 V
or 2.5 V, and series terminated transmission line or capacitive
loading. Depending on a given set of these operating
conditions and the available device convection, a decision on
the maximum operating frequency can be made.
Table 7. Die Junction Temperature and MTBF
Junction Temperature (C) MTBF (Years)
100 20.4
110 9.1
120 4.2
130 2.0
Table 8. Thermal Package Impedance of the 48ld LQFP
Convection, LFPM
R
thja
(1P2S board), K/W
Still air 78
100 lfpm 68
200 lfpm 59
300 lfpm 56
400 lfpm 54
500 lfpm 53
P
TOT
= [ I
CCQ
+ V
CC
· f
CLOCK
· ( N · C
PD
+ C
L
) ] · V
CC
M
P
TOT
= V
CC
· [ I
CCQ
+ V
CC
· f
CLOCK
· ( N · C
PD
+ C
L
) ] + [ DC
Q
· I
OH
· (V
CC
– V
OH
) + (1 – DC
Q
) · I
OL
· V
OL
]
M
P
T
J
= T
A
+ P
TOT
· R
thja
f
CLOCK,MAX =
C
PD
· N · V
2
CC
1
· [
– (I
CCQ
· V
CC
) ]
R
thja
T
j,MAX
– T
A
Equation 1
Equation 2
Equation 3
Equation 4
